The claimed invention relates generally to the field of electrical circuits and more particularly, but not by way of limitation, to a load balancing circuit used to balance output voltages from a dual polarity power supply with single polarity voltage regulation.
Electrical circuits require source voltages at appropriate potentials with respect to a reference level (such as ground). Such is the case for the electrical circuits of a disc drive, which is a digital data storage device that enables a user of a computer system to store and retrieve digital data in a fast and efficient manner.
A typical disc drive requires a variety of operational voltage input levels, such as positive five and twelve volts (+5 and +12V) and negative five volts (xe2x88x925V). These voltages are externally supplied to the disc drive by a computer power supply. A typical disc drive further has one or more internal power supplies that generate various voltage levels used by the disc drive during operation. For example, small motors (microactuators) used to provide fine positioning of disc drive recording heads adjacent disc recording surfaces can require the application of relatively large source voltages, such as xc2x140 volts.
A dual polarity power supply provides one output voltage having a positive potential with respect to a reference level and another output voltage having a negative potential with respect to the reference level. The output voltages may or may not be mirrored about the reference level (i.e., the output voltages might be xc2x115V, or the output voltages might be +20V and xe2x88x9210V). Voltage regulation circuitry provides a feedback control loop to adjust the operational characteristics of the power supply to maintain the output voltages at nominal levels.
A dual polarity power supply can have single polarity (single sided) regulation or dual sided regulation. Dual sided regulation involves providing separate regulation circuits for each output voltage. The regulators work independently to maintain the respective output voltages at the desired output levels. Single sided regulation involves providing a regulation circuit on just one of the output voltages. The output voltages are configured to operate in a lead-follow fashion so that unregulated output voltage will tend to follow adjustments made to the regulated output voltage.
While dual sided regulation generally provides better voltage regulation performance, dual sided regulation supplies are also more expensive and complex to implement. In the cost sensitive environment of disc drives, manufacturers often attempt to implement dual polarity power supplies with single sided regulation and rely on the ability of the single regulator to maintain both polarities of output voltage at near nominal operating levels.
Single sided regulation has been found to provide adequate performance as long as the respective positive and negative loads supplied by the power supply remain within a certain amount of relative balance. However, when large changes occur in the amount of impedance presented by one of the loads, the single regulator may not be able to maintain both output voltages at desired operational levels and voltage drift may result. Regulation of the positive voltage with a lower impedance load on the positive output than on the negative output can cause the negative output voltage to increase with respect to the positive output voltage. Conversely, lower impedance on the negative output than on the positive output causes the negative output voltage to decrease with respect to the positive output voltage.
Disc drive manufacturers have generally implemented dual polarity power supplies with single sided regulation in applications where the respective loads do not vary a significant amount over time, and have resorted to the more expensive and complex dual sided regulation power supplies for applications involving relatively large amounts of load variance.
There is a need, therefore, for improvements in the art to enable a dual polarity power supply with single polarity regulation to provide adequately maintained positive and negative output voltages for loads that undergo significant impedance changes over time.
In accordance with preferred embodiments, a load balancing circuit is provided to balance the output voltages of a dual polarity power supply with single polarity regulation. The power supply outputs a positive voltage at a positive voltage with respect to a reference level and a negative voltage with respect to the reference level. The power supply further applies voltage regulation to a selected one of the positive and negative voltages and leaves the remaining one of the positive and negative voltages in an unregulated state. The remaining, unregulated voltage relies on the regulation of the regulated voltage to maintain the unregulated voltage at a desired nominal level.
Positive and negative loads are coupled to receive the positive and negative voltages. Preferably, the positive and negative loads compose a microactuator in a disc drive which provides fine positioning of a read/write head with respect to a disc recording surface.
The load balancing circuit detects a change in impedance of a selected one of the positive and negative loads and which applies a corresponding impedance to the power supply. This additional impedance serves to balance the loads sensed at the output terminals of the power supply, thereby maintaining the power supply in a nominally balanced operational state. Preferably, the load balancing circuit comprises a summing and comparing network which generates an adjustment signal in relation to a difference between the reference level and a sum of the positive voltage and a negative voltage, and an impedance network which applies the corresponding impedance in relation to the adjustment signal.
These and various other features and advantages which characterize the claimed invention will be apparent from a reading of the following detailed description and a review of the associated drawings.